Discontinuous deformation analysis (DDA) was created by Shi & Goodman. It solves a finite element type of mesh where all the elements are isolated blocks and bounded by pre--exisfing discontinuities under kinematic conditions of dynamic and quasi-static motion. The authors introduced an elasto-plasfic yield criterion in the analysis, and added several new elements to handle with practical rock mechanics problems. The results show validity of the method for practical use.

A new numerical methoddiscontinuous deformation analysis (DDA) was invented by Shi & Goodman (1984, 1985) and further developed by Shi & Goodman (1988, 1989)since then. This method uses the displacements and strains as unknown variables in an element block, and solves the equilibrium equations in the same manner as the matrix analysis of structures in the Finite Element Method.

The original DDA only uses an elastic material property for a block and the friction is activated along a block-to-block interface. In this paper improvements of the original DDA are described and the new code is applied to solve rock stability problems. The block element in the new code can deform as an elasto-plastic material following the DruckerPrager associated constitutive law. The main purpose to take into account the block yielding is to analyze soft rock mass behavior subjected to various loading conditions such as excavation and embankment. The interface between blocks behaves according to the Mohr-Coulomb's criterion including cohesive force. Damping coefficient was implemented to take into account the block collision. The rockbolt element was introduced to represent the effect of confinement for rock masses. The bonding element which fuses two blocks was also invented to represent shotcrete in a tunnel structure.

The new DDA code was calibrated in comparison with laboratory model tests. Stability of rock slope and tunnel in a discontinuous rock mass is analyzed, and the effect of lining and rock bolts is discussed. Rockfall on a very steep slope was also calculated. Some of the results were compared with those of the finite element method. The method of DDA with improvement has been proven to be very effective to analyze rock stability problems in discontinuous deformable rock masses.

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